Stage 17 #2: Connecting indicator + auto-retry, instead of red toasts

Connectivity failures become state, not a toast on every attempt. A global online
signal (lib/connection.svelte.ts) flips on a transport unavailable / rate_limited and
on the live stream's drop, driving a pure-CSS header spinner + 'Connecting…' in place
of the title and softly disabling the in-game server actions (commit / exchange / pass
/ hint; local board/rack/reset stay live).

- transport: exec auto-retries with capped exponential backoff — every op on a
  rate-limit (rejected before processing, safe), reads only on unavailable (a mutation
  is never blindly re-sent, to avoid double-applying one whose response was lost; its
  button is disabled while offline so the player re-issues on reconnect). A reachability
  watcher (profile.get probe) and any successful traffic clear the signal.
- the old red error.unavailable toast is gone (handleError suppresses connection codes;
  the indicator replaces it). A server-data screen still opens with the spinner and
  fills on reconnect (global indicator + read auto-retry), so navigation is never dead.
- pure retry policy unit-tested (retry.ts); a mock-only window.__conn hook drives a
  Chromium+WebKit e2e (indicator shows offline, the action disables, both clear on
  reconnect). Full suite + build green.
- docs: ARCHITECTURE transport note, FUNCTIONAL (+ _ru), PLAN tracker (incl. #1 — the
  bot already drains all updates, no change).

Also records #1 as investigated/no-change in PLAN. Other server-action buttons (chat
send, profile save, …) still degrade to a safe no-op offline; visual disable is easy to
extend.

docs/FUNCTIONAL.md

@@ -46,7 +46,12 @@ request) arrive as a **Telegram notification** instead — unless the player kee
 notifications in the app only (a profile setting, **on by default**). The "your turn"
 notification names the opponent and recaps their last move — the word and the running score
 for a scoring play, or that they swapped or passed — and a finished game sends a "game over"
-notification with your result and the final score (scores read with yours first).
+notification with your result and the final score (scores read with yours first). If the
+connection drops or the server is rate-limiting, the app does not nag with errors: the header
+shows a quiet **"Connecting…"** spinner while it reconnects, actions that send to the server
+pause until it is back (a server-data screen still opens, with the spinner, and fills in on
+reconnect), and pending reads resume on their own — the interface stays usable instead of
+flashing a red banner each time.
 
 ### Accounts, linking & merge *(Stage 1 / 11)*
 First platform contact auto-provisions a durable account. From the profile a player